Frequently Asked Questions: Calculating Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) RT-PCR Laboratory Test Percent Positivity [open pdf - 623KB]
From the Document: "Percent positivity represents the percentage of all COVID-19 [coronavirus disease 2019] ([SARS-CoV-2 [severe acute respiratory syndrome coronavirus 2] RT-PCR [reverse transcription polymerase chain reaction]) tests conducted that are positive. While the methods used to calculate percent positivity can differ, percent positivity provides important insights into the transmission of an infectious disease, such as COVID-19 (SARS-CoV-2), in a geographical area (e.g., national, regional, state, county). Percent positivity provides a strong indication of how widespread infection is in an area where testing is being conducted, but is dependent upon whether testing is keeping up with the level of disease transmission and the criteria used for testing (routine screening vs. diagnostic testing)."
Centers for Disease Control and Prevention: https://www.cdc.gov/